DISEASES OF AQUATIC ORGANISMS Vol. 48: 57–74, 2001 Published December 20 Dis Aquat Org REVIEW Diseases, prophylaxis and treatment of the Atlantic halibut Hippoglossus hippoglossus: a review Øivind Bergh*, Frank Nilsen, Ole B. Samuelsen Institute of Marine Research, Department of Aquaculture, PO Box 1870 Nordnes, 5817 Bergen, Norway ABSTRACT: After substantial investments in research, the Atlantic halibut Hippoglossus hippoglos- sus is now being cultivated commercially in Norway, Iceland, Scotland and Canada. As with other domesticated species, disease problems have been experienced. This review summarizes the current state of knowledge of diseases of the Atlantic halibut, and their diagnosis, prophylaxis and treatment. In economic terms, the most important losses have been suffered at the larval and juvenile stages. The most important infections are caused by nodaviruses, causative agents of Viral Encephalopathy and Retinopathy (VER), which are the major reason why Norway’s production of halibut fry has been level since 1995. An aquatic birnavirus, Infectious Pancreatic Necrosis Virus, is also an important agent of mortality. Vibrio anguillarum, Flexibacter ovolyticus and atypical Aeromonas salmonicida are the major bacterial pathogens. The protozoan parasites recorded include Ichthyobodo sp., the microsporidium Enterocytozoon sp., Trichodina hippoglossi, and the metazoan pathogens include myxozoans, helminths, Entobdella hippoglossi, Lepeophtheirus hippoglossi and other parasitic cope- pods. Experimental vaccines have been tested against V. anguillarum and atypical A. salmonicida, with good results. A recombinant vaccine against nodaviruses is under development. A few trials have been carried out on non-specific immunostimulants, but no such treatment is currently avail- able. A number of efficacy and pharmacokinetic trials with various antibacterial agents have also been published. KEY WORDS: Atlantic halibut · Diseases · Prophylaxis · Treatment Resale or republication not permitted without written consent of the publisher INTRODUCTION cies (Tilseth et al. 1990). Following the commercial success of the salmon industry in Norway and Scot- During the late 1980s and the 1990s, substantial land, and due to its high price, well-established mar- efforts have been made in several countries to bring kets, and its natural presence in cold waters, the hali- the Atlantic halibut Hippoglossus hippoglossus into but seemed to be a good candidate for aquaculture in commercial aquaculture (reviewed by Mangor-Jensen Northern Europe and Canada. et al. 1998b). The natural habitat of the halibut is the The domestication of a new species inevitably in- waters of the North Atlantic Ocean and the Norwegian volves disease control, and there is no doubt that this Sea (Haug 1990). It was therefore thought that the has also been experienced in halibut aquaculture, for a natural conditions off the coasts of Norway, Scotland, number of reasons. The high host density in aquacul- Iceland and Canada would be favourable for the spe- ture gives parasitic organisms a competitive ecological advantage compared to the natural situation. Sec- ondly, suboptimal rearing conditions resulting from at- *E-mail: [email protected] tempts to cultivate organisms whose biology is largely © Inter-Research 2001 · www.int-res.com 58 Dis Aquat Org 48: 57–74, 2001 unknown may accidentally compromise the defence of potential harmful parasites in farmed fish is limited. systems of the host. Thirdly, disease conditions that Studies of parasites on wild halibut have mainly con- may be common among wild fish are largely unrecog- centrated on describing metazoan parasites, particu- nised by humans. However, in aquaculture systems the larly helminths (e.g. Polyanskii 1955, Scott & Bray fish farmer will recognise signs of disease such as 1989). Furthermore, all these studies were performed reduced growth, abnormal behaviour and increased on larger halibut (from 29 cm length upwards) and mortality, all of which rapidly call for attention by there are no published data on parasites on wild veterinarians and scientists. halibut fry. The main challenge experienced by halibut farms In aquaculture, parasites with a direct life cycle have has been to achieve adequate survival rates, particu- the greatest potential to be causative agents of dis- larly during the early life stages. Thus, since the early eases. The low occurrence or absence in aquaculture 1990s, the amount of experience and knowledge of dis- of intermediate hosts needed for transmission for many eases of viral, bacterial or parasitical etiology of the groups of parasites excludes parasites with more com- Atlantic halibut has grown significantly. The evidence plicated life cycles. However, the production of marine for the importance of diseases that affect the early life fish normally requires the use of zooplankton as start stages, such as infections by birnaviruses (Biering et al. feed. By using wild-caught zooplankton, potential in- 1994), nodaviruses (Grotmol et al. 1995, 1997), or vari- termediate hosts are introduced as feed for the larvae. ous opportunistic bacteria (Bergh et al. 1992b) has now Several groups of common parasites are transmitted been demonstrated by different challenge experiments through zooplankton, including the digenea, cestoda, (Table 1). nematoda and acatocephala. Most of these parasites The parasite fauna of wild Atlantic halibut have not show a low degree of pathogenity to larger fish but been widely studied, for which reason our knowledge larvae and fry are considerably more vulnerable. The objective of this paper is to pro- Table 1. Microbial pathogens known to cause mortality in experimental vide an overview of the various dis- challenge studies with different developmental stages of Atlantic halibut. eases of Atlantic halibut, including IPNV: Infectious Pancreatic Necrosis Virus reports on prophylactic and therapeu- tic procedures used to control these Ontogenetic stage Pathogen Source diseases. Eggs and Flexibacter ovolyticus Bergh et al. (1992b) yolk-sac larvae Vibrio anguillarum V. salmonicida COMMERCIAL HALIBUT V. splendidus PRODUCTION Eggs and Aeromonas salmonicida subsp. Bergh et al. (1997) yolk-sac larvae salmonicida Apart from some pioneering experi- Eggs and F. ovolyticus Bergh (2000) ments by Rollefsen (1934), who re- yolk-sac larvae (3 different strains) ported successful hatching of halibut A. salmonicida subsp. salmonicida eggs in his laboratory, the first V. anguillarum attempts to rear Atlantic halibut, lar- Weaned fry IPNV Biering et al. (1994) vae for aquacultural purposes were (0.1, 1.0 and 3.5 g) made in Norway between 1974 and Yolk-sac larvae IPNV Biering & Bergh (1996) 1980 (Blaxter et al. 1983). As a result, the first 2 metamorphosed fry were Subadults A. salmonicida Bricknell et al. (1999) subsp. salmonicida produced in 1980 in floating plastic Atypical A. salmonicida bags at Flødevigen Research Station, Yolk-sac larvae Nodavirus Grotmol et al. (1999) Arendal, Norway. This stimulated a (halibut strain) research programme on halibut culti- Fry (15 g) Atypical A. salmonicida Ingilæ et al. (2000) vation in Norway, which was followed Fry (<5 g) V. anguillarum Samuelsen (1997) by similar programmes in the UK, Ice- land and Canada (Mangor-Jensen et Fry (<5 g) V. anguillarum Samuelsen et al. (1997a) al. 1998b). Yolk-sac larvae V. anguillarum Skiftesvik & Bergh (1993) Trial commercial production of the F. ovolyticus Atlantic halibut is currently underway Yolk-sac larvae Nodavirus Totland et al. (2000) in these 4 countries (Fig. 1). In 1999 (halibut strain) (striped jack strain) the total production of fry in Norway was estimated at 346 000 individuals, Bergh et al.: Disease of Atlantic halibut: a review 59 may occur in captivity, the usual method entails strip- ping eggs and milt from ripe broodstock, followed by artificial fertilization (Mangor-Jensen et al. 1998a). Halibut eggs are transparent and naturally buoyant at a salinity of 32 to 34 ppt. Eggs are normally reared in upwelling incubators, with a plankton net to prevent them from being drained out at the overflow. The opti- mum water temperature limits are relatively narrow at 5 to 7°C. The eggs hatch approximately 82 day- degrees after fertilization when incubation tempera- tures are kept within these limits (Mangor-Jensen et al. 1998a). Seasonally independent egg and fry pro- duction has been achieved for halibut by the photope- riod manipulation of broodstock (Næss et al. 1996). At hatching, the larva is at a primitive ontogenetic stage (Pittman et al. 1990b). The eyes are poorly devel- Fig. 1. Hippoglossus hippoglossus. Annual production of indi- oped, and electron microscopic observations have viduals of juvenile Atlantic halibut in Norway, Iceland, Scot- land and Canada 1988–2000. Data extracted from van der shown that the retina does not become functional until Meeren (2000) and the NUMARIO programme of the Nor- 150 degree-days after hatching (Kvenseth et al. 1996). wegian Research Council Light is not a critical factor for normal morphogenesis of the retina of halibut to the time of first feeding (Helvik & Karlsen 1996), and yolk-sac larvae are there- in Iceland 350 000, in Scotland 170 000 and Canada fore typically reared in darkness. The mouth and anus 48 000 (van der Meeren 2000). According to informa- are not open at hatching, and the only access from the tion from halibut farmers, the production in 2000 environment to the intestine is through the pseudo- increased in Iceland to 450 000 and in Canada